Usecounter for AppCache installed pages with no manifest

The very fact that AppCache is supported introduces a delay to preloaded
requests. We could use the `manifest` attribute to detect pages where
AppCache is used and only delay requests there.
This CL adds a counter for pages where this heuristic will fail (and
likely result in a spurious download). Assuming the counter is
relatively low, we'd be able to go ahead with implementing the
heuristic.

Bug: 788757
Change-Id: If5ca0c8682fa677677df38f59946b3198588d4a3
Reviewed-on: https://chromium-review.googlesource.com/c/1442051
Commit-Queue: Yoav Weiss <yoavweiss@chromium.org>
Reviewed-by: Mike West <mkwst@chromium.org>
Cr-Commit-Position: refs/heads/master@{#626968}

third_party/blink/public/platform/web_feature.mojom

@@ -2204,6 +2204,7 @@ enum WebFeature {
   kHTMLTemplateElement = 2769,
   kNoSysexWebMIDIWithoutPermission = 2770,
   kNoSysexWebMIDIOnInsecureOrigin = 2771,
+  kApplicationCacheInstalledButNoManifest = 2772,
 
   // Add new features immediately above this line. Don't change assigned
   // numbers of any item, and don't reuse removed slots.

third_party/blink/renderer/core/html/html_html_element.cc

@@ -84,11 +84,20 @@ void HTMLHtmlElement::MaybeSetupApplicationCache() {
     return;
   }
 
+  ApplicationCacheHost* host = document_loader->GetApplicationCacheHost();
+  DCHECK(host);
+
   if (manifest.IsEmpty())
-    document_loader->GetApplicationCacheHost()->SelectCacheWithoutManifest();
+    host->SelectCacheWithoutManifest();
   else
-    document_loader->GetApplicationCacheHost()->SelectCacheWithManifest(
-        GetDocument().CompleteURL(manifest));
+    host->SelectCacheWithManifest(GetDocument().CompleteURL(manifest));
+  bool app_cache_installed =
+      host->GetStatus() !=
+      blink::mojom::AppCacheStatus::APPCACHE_STATUS_UNCACHED;
+  if (app_cache_installed && manifest.IsEmpty()) {
+    UseCounter::Count(GetDocument(),
+                      WebFeature::kApplicationCacheInstalledButNoManifest);
+  }
 }
 
 }  // namespace blink

tools/metrics/histograms/enums.xml

@@ -21468,6 +21468,7 @@ Called by update_net_error_codes.py.-->
   <int value="2769" label="HTMLTemplateElement"/>
   <int value="2770" label="NoSysexWebMIDIWithoutPermission"/>
   <int value="2771" label="NoSysexWebMIDIOnInsecureOrigin"/>
+  <int value="2772" label="ApplicationCacheInstalledButNoManifest"/>
 </enum>
 
 <enum name="FeaturePolicyFeature">